The present invention relates to a supercapacitor. In particular, it relates to a supercapacitor, the major energy storage method of which is based on the reaction of electrochemical active materials in the thin liquid layer near to the inner and outer surface of the porous electrodes.
As the human society develops it has become more urgent to develop clean, highly efficient ways to utilize energy for the sustainable development of society. Fixed users can easily access the power supply network, whereas mobile users (such as automobiles) have to be dependent on energy storage devices. Such energy storage systems mainly include fossil fuel (for fuel vehicles) or storage battery (for electrically-propelled vehicles) listed below.
Because the caloric value is very high for gasoline, it ranks on top of the list for the highest energy density in term of chemical energy storage. Therefore, the speed and mileage for fossil fuel vehicles are advantageous. The drawbacks of fuel automobile, however, (especially in urban areas) are becoming very serious with the depletion of petroleum resources gasoline and the worsening of the environmental pollution caused by burning fossil fuels. Therefore, the need to develop vehicles with clean and economic energy is also becoming increasingly urgent.
Electrically-propelled automobiles with storage a battery or batteries as energy storage devices do not pollute the environment. However, the storage batteries have the disadvantages of a lower power density, bad cycle life and safety problems, and cannot meet the desired power requirements. In recent years, greater efforts have been made to develop supplementary energy with higher specific power and moderate specific energy as energy storage devices for hybrid electric automobiles, aiming to meet the requirements of high power for electrically-propelled automobiles for accelerating, starting and braking.
Electrically-propelled automobiles may also use supercapacitors as an energy storage device. Because the supercapacitors possess the advantages of (1) rapid charging and discharging, (2) no environmental pollution, (3) a longer cycle life, and (4) the ability to recover the kinetic automobile braking energy, they promise to be the new urban green energy for this century. The capacity of supercapacitors first comes from the capacity of the electric double layer on an electrode/solution interface. By using the high specific surface of carbon material and an aerogel, a bigger capacitor with a capacity of up to 10-100 F/g can be generated, and the charging and discharging speed is typically very fast. But compared with storage batteries, the energy density of the supercapacitor with the electric double layer capacity is low. The second source of supercapacitor capacity comes from the electrochemical process on the material surface, which is also referred to as “pseudo-capacity.” The presence of the pseudo-capacity could increase greatly the capacity of supercapacitors. The pseudo-capacitor is also essentially determined by the electrochemical process for energy storage, which is fundamentally the same as the storage battery (such as, for example, a lithium battery) and, therefore, the supercapacitor based on the pseudo-capacity can be regarded as a battery with super high specific powder and long life. Currently, the means for the pseudo-capacity formation include:
(1) Chemical surface absorption and desorption as well as underpotential deposition on the electrode;
(2) Oxidation and reduction reaction of oxides membrane such as RuO2, IrO2 and Cr3O4 on the electrode surface;
(3) Doping and undoping of the electrically conductive polymer; however, the various pseudo-capacity supercapacitors have drawbacks such as, for example, a poor life cycle, high price, and other issues that prevent them from meeting the requirements of practical use.
There are many patents for supercapacitors. For an example, the recent U.S. Pat. No. 7,049,233 proposes to use porous ruthenium oxide membrane as an electrode. U.S. Pat. No. 6,671,166 discloses an organic supercapacitor made of carbon material with high specific surface. CN1770344 discloses a supercapacitor in which the medal oxide nanotube and porous carbon composite material are used as both cathode and anode. Such supercapacitors, however, all depend on solid surface electric double layer capacity or “pseudo-Faraday” reaction with a solid electrode as the active material. There is a need in the art for an improved supercapacitor.